The present invention relates to a scanning optical system employed in a laser beam printer or the like. More specifically, the present invention relates to a scanning optical system which includes a curved reflection surface to reflect a laser beam deflected by a polygonal mirror and to direct the reflected laser beam to a surface to be scanned.
Conventionally, a scanning optical system includes a polygonal mirror which deflects a beam emitted by a laser diode to scan a surface to be scanned (e.g., a photoconductive surface of a photoconductive drum), and an fxcex8 lens which converges the beam to form a scanning beam spot on the surface such that the scanning beam spot scans on the surface at a constant speed.
As the polygonal mirror rotates about its rotational axis, the beam spot moves on the surface to be scanned. Hereinafter, a direction, on the surface to be scanned, in which the beam spot moves as the polygonal mirror rotates is referred to as a main scanning direction. By ON/OFF modulating the beam spot as it moves in the main scanning direction, and by moving the surface to be scanned in a direction perpendicular to the main scanning direction, a two dimensional image can be formed on the surface. Hereinafter, the direction, on the surface to be scanned, perpendicular to the main scanning direction is referred to as an auxiliary scanning direction. Further, shape and direction of power of each optical element provided in the scanning optical system is described with reference to directions on the surface to be scanned.
A cylindrical lens which has a positive power only in the auxiliary scanning direction is also provided in the scanning optical system. The beam passed through the cylindrical lens is converged in the auxiliary scanning direction, and the converged beam is incident on a reflection surface of the polygonal mirror. Further, a power of the fxcex8 lens in the auxiliary scanning direction is determined so that the reflection surface of the polygonal mirror and the surface to be scanned have a conjugate relationship. With this configuration, a facet error of the polygonal mirror can be corrected.
Recently, an optical scanning system, which uses a reflector having a curved reflection surface (which will be referred to as an fxcex8 mirror hereinafter) in place of the fxcex8 lens, has been provided. With this type of scanning optical system, it is expected that chromatic aberration is reduced. It is also expected that the size of the optical scanning system is reduced because of the bent configuration of an optical path. In general, in such a scanning optical system, a polygonal mirror whose thickness is reduced in order to achieve weight reduction thereof is employed.
The fxcex8 mirror reflects the beam deflected by the polygonal mirror, and directs the beam to the surface to be scanned. The reflection surface of the fxcex8 mirror is typically an anamorphic surface, which is symmetrical with respect to an auxiliary scanning plane including the rotational axis of the polygonal mirror and being perpendicular to the surface to be scanned.
Similar to the fxcex8 lens, the fxcex8 mirror has functions of maintaining the constant scanning speed of the beam spot on the surface to be scanned, correcting curvature of field both in the main scanning direction and in the auxiliary scanning direction, and correcting the facet error of the polygonal mirror.
In the scanning optical system employing the fxcex8 mirror, the light source, the polygonal mirror, and the fxcex8 mirror are arranged such that the beam incident on the polygonal mirror travels in the auxiliary scanning plane, in order to prevent occurrence of aberration caused by the reflection surface of the fxcex8 mirror.
FIG. 1 shows a typical configuration of a conventional scanning optical system which uses an fxcex8 mirror 5. As shown in FIG. 1, a laser beam emerging from a cylindrical lens 3 is reflected by a polygonal mirror 4, and travels in the reverse direction. Then, the laser beam is reflected by the fxcex8 mirror 5, and is directed to a photoconductive drum 11.
Travel of the laser beam in the cylindrical lens 3 will be described in detail. As shown in FIG. 1, the laser beam emitted by a light source (not shown) enters the cylindrical lens 3 through a cylindrical surface 3a, exits from a planar surface 3b, and travels to the polygonal mirror 4 along an optical path Ax. Generally, a collimating lens is arranged between the light source and the cylindrical lens 3. Accordingly, a collimated laser beam is incident on the cylindrical lens 3.
When the laser beam emitted by the laser source is incident on the cylindrical surface 3a, most part of the laser beam passes through the cylindrical surface 3a and exits from the planar surface 3b. However, a remaining part of the laser beam is reflected by the planar surface 3b. Further, the part of the laser beam reflected by the planar surface 3b is partially reflected by the cylindrical surface 3a, thereby stray light G being generated and emerging from the planar surface 3b. 
Since the stray light G is converged twice by the cylindrical surface 3a, it converges on a point between the cylindrical lens 3 and the polygonal mirror 4, and is incident on the polygonal mirror 4 as a diverging beam.
As can be seen in FIG. 1, a portion of the stray light G is deflected by the polygonal mirror 4, and therefore, the portion of the stray light scans on the photoconductive drum 11. It is understood that the portion of the stray light G scanning on the photoconductive drum 11 does not affect the quality of an image formed on a photoconductive surface of the photoconductive drum 11 substantially because it is very weak and does not stay on the same position on the photoconductive surface.
On the contrary, most part of the stray light G passes by the polygonal mirror 4 and is directly incident on the photoconductive drum 11. That is because the thickness of the polygonal mirror 4 is reduced in order to achieve weight reduction thereof. The stray light G directly incident on the photoconductive drum 11 affects the quality of an image since, although it is very weak, the stray light G directly incident on the photoconductive drum 11 is not deflected by the polygonal mirror 4 and stays on the same position on the photoconductive surface.
The present invention is advantageous in that it provides a scanning optical system, which is capable of preventing deterioration of the quality of an image formed on a surface to be scanned due to the stray light generated by the cylindrical lens.
According to an aspect of the invention, there is provided a scanning optical system for emitting a beam scanning in a main scanning direction. The scanning optical system is provided with a light source that emits a beam, a cylindrical lens that converges the beam emitted by the light source in an auxiliary scanning direction which is perpendicular to the main scanning direction, a polygonal mirror that rotates and deflects the beam emerged from the cylindrical lens to scan in the main scanning direction, and an optical element that has a reflection surface to reflect the beam deflected by the polygonal mirror. The optical element is configured to converge the beam deflected by the polygonal mirror to form a beam spot on a surface to be scanned, and to enable the beam spot to scan on the surface to be scanned at a constant speed. In this case, the cylindrical lens is arranged such that a central axis of the beam, which enters into the cylindrical lens and exits from the cylindrical lens without being reflected by inner surfaces of the cylindrical lens, is deflected in the auxiliary scanning direction by the cylindrical lens and is incident on the polygonal mirror.
With this configuration, it becomes possible to prevent a portion of the light beam, which is reflected by the inner surfaces of the cylindrical lens a plurality of times and exits from the cylindrical lens, from being directed to the surface to be scanned.
In a particular case, an optical axis of the cylindrical lens may be shifted in the auxiliary scanning direction with respect to the central axis of the beam traveling from the cylindrical lens to the polygonal mirror.
In a particular case, a central axis of a portion of the beam, which is reflected by inner surfaces of the cylindrical lens a plurality of times and exits from a side of the cylindrical lens facing the polygonal mirror, may be inclined in the auxiliary scanning direction with respect to the beam traveling from the cylindrical lens to the polygonal mirror.
Optionally, the scanning optical system may be provided with a light shield that is placed on an optical path of the portion of the beam and blocks travel of the portion of the beam.
Preferably, the central axis of the beam traveling from the cylindrical lens to the polygonal mirror may be inclined with respect to a plane which is perpendicular to a rotational axis of the polygonal mirror.
Optionally, the optical element may be positioned between the cylindrical lens and the polygonal mirror when the scanning optical system is viewed from a line parallel with the rotational axis of the polygonal mirror, and may be placed at a distance from the beam traveling from the cylindrical lens to the polygonal mirror.
In a particular case, the cylindrical lens may be arranged such that a cylindrical surface thereof is facing the light source.
Optionally or alternatively, the cylindrical lens may have a cylindrical surface and a planar surface.
In a particular case, the reflection surface of the optical element may be formed on a rear side of the optical element. In this case, the rear side is opposite to a front side of the optical element facing the polygonal mirror.
In a particular case, stray light may be generated inside the cylindrical lens due to inner reflection thereof. In this case, the stray light is emitted from the cylindrical lens on the same side where the beam directed to the polygonal mirror emerges, and the cylindrical lens is arranged so that the stray light is spatially separated from the beam directed to the polygonal mirror.
According to another aspect of the invention, there is provided a scanning optical system, which is provided with a light source that emits a beam, a cylindrical lens that converges the beam emitted by the light source in an auxiliary scanning direction, a polygonal mirror that deflects the beam from the cylindrical lens to scan within an predetermined angular range, and an fxcex8 mirror that converges the scanning beam on a surface to be scanned. In this case, stray light is generated inside the cylindrical lens due to inner reflection thereof, and the stray light is emitted from the cylindrical lens on the same side where the beam directed to the polygonal mirror emerges. Further, the cylindrical lens is arranged so that the stray light is spatially separated from the beam directed to the polygonal mirror.
Optionally, an optical axis of the cylindrical lens may be shifted with respect to a central axis of the laser beam incident on the cylindrical lens.
Still optionally, an optical axis of the cylindrical lens may be inclined with respect to a central axis of the laser beam incident on the cylindrical lens.
In a particular case, a degree of separation of the stray light with respect to the beam directed to the polygonal mirror may be changeable depending on the arrangement of the cylindrical lens.
In a particular case, the scanning optical system may include a light shielding member which blocks the stray light.